A federal judge has said Hillary Clinton may be required to testify about her private email system while secretary of state as part of a freedom of information act lawsuit brought by a conservative watchdog group.
It was the latest twist in a long-running email saga that continues to dog Clinton as she runs for president as the presumptive Democratic nominee.
Judge Emmet Sullivan on Wednesday gave the go ahead for Judicial Watch to take testimony from several of Clinton's close aides when she was secretary of state.
"Based on information learned during discovery, the deposition of Mrs Clinton may be necessary," Sullivan said in an order granting Judicial Watch discovery.
The order authorizes depositions of seven former State Department officials about the private email system Clinton used for much of her electronic correspondence while secretary of state.
They include Cheryl Mills, who was Clinton's chief of staff at the State Department; Huma Abedin, her former deputy chief of staff; and Bryan Pagliano, the State Department employee who reportedly set up the email system.
Clinton's use of a private server for both official and private correspondence first came to light in 2015 during Republican-led congressional investigations into her handling of a militant attack on the US mission in Bengazi, Libya.
The assault in 2012 left the US ambassador and three other Americans dead.
The FBI has since launched a criminal investigation amid Republican charges that use of the unsecured system endangered national security.
Judicial Watch's tie-in to the controversy is a freedom of information act it had filed in 2013 seeking information about Abedin's employment by the State Department under a special status that allowed her to work for others outside the department while serving as Clinton's adviser.
It dropped the suit a year later after receiving assurances from the State Department that it had searched for the requested records.
But after learning of Clinton's private email server, Judicial Watch reopened the suit, alleging that the State Department had not acted in good faith because its records search did not include the secretary's emails.